DOPAMINE NEURON MEMBRANE PHYSIOLOGY - CHARACTERIZATION OF THE TRANSIENT OUTWARD CURRENT (I-A) AND DEMONSTRATION OF A COMMON SIGNAL-TRANSDUCTION PATHWAY FOR I-A AND I-K

Dopamine neurons derived from the mesencephalon of embryonic rats were maintained in primary culture, identified and studied with whole-cell patch recording techniques. These neurons demonstrated a rapidly activating and inactivating voltage-dependent outward current which required the presence of K+ ions. This current was termed I-A because of its transient nature. It was elicited by step depolarizations from holding potentials more negative than -50 mV and exhibited steady-state inactivation at a membrane potential more positive than -40 mV and half-maximal inactivation observed at -65 mV. This current rapidly achieved peak activation in less than 8 msec and decayed with a time constant (tau) Of 58 +/- 5 msec. This current was observed in the presence of tetraethylammonium but was readily blocked by 4-aminopyridine (2-4 mM). This current was also observed to be modulated by stimulation of D-2 dopamine receptors (DA autoreceptors) located on the dopamine neurons. Thus, both DA and the D-2 receptor agonist quinpirole enhanced the peak I-A observed, while the partial D-1 receptor agonist SKF 38393 was without effect. The enhancement of I-A was confirmed to be due to the activation of D-2 receptors as the effects of either DA or quinpirole were blocked by the D-2 receptor antagonists eticlopride and sulpiride, but not by the D-1 receptor antagonist SCH 23390. Since we have previously demonstrated that the I-K present in these cells is also enhanced by D-2 receptor stimulation, we investigated the signal transduction pathways involved in coupling DA autoreceptors to both I-A and I-K. The response of both these potassium currents to DA autoreceptor stimulation was completely abolished by the preincubation of cultures with pertussis toxin, indicating the possible involvement of the G proteins G(i) and G(o). In an attempt to further characterize which G protein may be involved, additional experiments were performed. The ability of DA autoreceptor stimulation to augment both currents was also blocked completely when G protein activation was prevented by the intracellular application of GDP beta S (100 mu M). In contrast, irreversible activation of G proteins by intracellular application of the nonhydrolyzable GTP analog GTP gamma S (100 mu M) mimicked the effects of DA autoreceptor stimulation on both I-A and I-K. In addition, the intracellular application of a polyclonal antibody that was selective for the alpha-subunit of G(o) completely abolished the DA autoreceptor modulation of both currents while preimmune serum was without effect. Taken together, these data demonstrate that the enhancement of I-A and I-K in response to stimulation of DA autoreceptors is dependent upon the activation of G(o) and appears to involve a G(o alpha) subunit. (C) 1994 Wiley-Liss, Inc.